1. Field of the Invention
The present invention relates to a photoelectric conversion device having signal lines for activating plural photoelectric conversion cells.
2. Related Background Art
FIGS. 1(A) to 1(C) illustrate photoelectric conversion cells in a conventional photoelectric conversion device.
FIG. 1(A) is a schematic plan view of a photoelectric conversion cell in a conventional photoelectric conversion device; FIG. 1(B) is a cross-sectional view along a line I--I in FIG. 1(A); and FIG. 1(C) is a cross-sectional view along a line II--II in FIG. 1(A).
As shown in these drawings, the photoelectric conversion cells are arranged on an n.sup.- layer 1 formed by epitaxial growth on a substrate, with an element separating area composed of an oxide film 2 formed by the LOCOS method.
On the n.sup.- layer 1 there is formed a p-base area 3 of a bipolar transistor, in which formed is an n.sup.+ emitter area 4. On oxide films 2, 5 there is formed a horizontal signal input line 6 which forms, on the p-base area 3, a capacitor electrode for controlling the potential of the base across the oxide film 5. The horizontal signal input line 6 is composed of polysilicon.
In addition, across an insulating layer 7 there is formed a vertical signal output line 8, which is composed of aluminum and is connected to an n.sup.+ -emitter area 4. In the insulating layer 7 there is formed a contact hole 9a through which the aluminum layer 9 is connected to the horizontal signal input line 6 composed of polysilicon. As the vertical signal output line 8 and the aluminum layer 9 are formed on the insulating layer 7, they will hereafter be called first aluminum layers.
After the formation of an insulating layer 10, a hole 9b is formed on the aluminum layer 9, and an aluminum layer 11 formed parallel to the horizontal signal input line 6 is connected with the aluminum layer 9. The uppermost aluminum layer 11 is electrically connected therewith through the aluminum layer 9.
The above-explained double-layered structure including the aluminum layer 11 of low resistance is adopted for preventing the signal proposition delay, resulting the higher resistance of the horizontal signal input line 6.
The aluminum layer 11, being formed on said first aluminum layer across the insulating layer 10, will be called the second aluminum layer.
FIG. 2 is a schematic view of a conventional photoelectric conversion device consisting of a plurality of the above-explained photoelectric conversion cells.
The above-explained photoelectric conversion cells C1, C2, . . . are arranged as an array, and the horizontal signal input line 6 and and the aluminum layer 11 are formed in a parallel manner and are connected to the cells through the aluminum layer 9. Consequently the horizontal signal can be transmitted without delay even in the case of a large number of cells, so that the signal read-out operation, refreshing operation etc. can be achieved at a high speed.
However, the above-explained conventional photoelectric conversion device has been associated with a drawback that the aperture ratio cannot be increased, because the aluminum layer 9 constituting the connection portion in the above-explained double-layered structure occupies about 11% of each cell area. The aperture ratio is the ratio between the effective photo-receiving area of the photoelectric device and the whole area thereof. In conventional devices, since the aluminum layer 9 serving as the connecting portion is opaque to light, the conventional structure having the aluminum layer 9 in each cell cannot increase the aperture ratio.
Such drawback is not limited to the above-explained conventional structure but appears also in various photoelectric conversion devices such as of CCD, MOS or SIT structure, as long as the double-layered structure is employed.